Single sideband electro-optic modulators have applications in coherent laser radar and in high resolutions spectroscopy. Optical single sideband generation can be achieved by phase velocity matching a circularly polarized (CP) optical field with a circularly polarized microwave field in a electro-optic (EO) crystal exhibiting 3-fold crystallographic symmetry The microwave field generates an optical indicatrix which rotates at half the microwave frequency. The rotating optical indicatrix, which is analogous to an optical field propagating through a spinning waveplate, produces a single sideband, up-shifted or down-shifted signal dependent on the rotation sense of the two fields at a frequency equal to twice the optical indicatrix angular rotation frequency. A passively achromatic quarter waveplate over the wave length range of interest is used to generate a circularly polarized optical wave.
Broadband frequency shifting of a laser beam at high speeds requires traveling-wave electro-optic phase modulation of the optical carrier frequency at microwave frequencies. Known optical modulators of this type are characterized by a conventional square waveguide structure filled with a cadmium telluride (CdTe) crystal. Generally, high levels of microwave power are needed.
To establish the required electric field strength with reduced poWer levels, the microwave energy must be confined to the smallest possible cross-sectional area and be concentrated in the electro-optic crystal. In addition, to convert the laser power into only one sideband at microwave frequencies, a near synchronous traveling-wave interaction with the optical beam is required. Thus, the modulator structure must provide a good velocity match that includes having a cut-off frequency well below that of the desired frequencies to minimize dispersion effects. Moreover, the overlapping of a Gaussian optical field with a near uniform microwave field must be perfect in order to avoid power transfer into other, unwanted sidebands. A suitable modulator structure must also have a 2-fold symmetry in order to establish a circularly polarized microwave field with uniformly distributed intensity within the interaction region.